io-ts

What is io-ts?

The io-ts npm package is a TypeScript library that allows for the definition of runtime types, and the automatic validation of runtime values against those types. It leverages TypeScript's type system to ensure that data structures conform to specified schemas, providing a bridge between the runtime data and compile-time types.

What are io-ts's main functionalities?

Runtime type validation

This feature allows you to define a type and then validate an object against that type at runtime. If the object matches the type, the 'Right' branch is executed; otherwise, the 'Left' branch indicates a validation error.

{"const t = require('io-ts');\nconst User = t.type({\n  name: t.string,\n  age: t.number\n});\nconst result = User.decode({ name: 'Alice', age: 25 });\nif (result._tag === 'Right') {\n  console.log('Valid!', result.right);\n} else {\n  console.log('Invalid!', result.left);\n}"}

Type composition

io-ts allows for the composition of types, enabling complex type definitions by combining simpler ones. This is useful for building up the shape of data structures from reusable type components.

{"const t = require('io-ts');\nconst Name = t.string;\nconst Age = t.number;\nconst User = t.type({ name: Name, age: Age });\nconst result = User.decode({ name: 'Bob', age: 'not-a-number' });\n// result will be an instance of Left since 'age' is not a number"}

Custom types

io-ts allows the creation of custom types with additional validation logic. In this example, a 'PositiveNumber' type is created that only accepts positive numbers.

{"const t = require('io-ts');\nconst PositiveNumber = t.brand(\n  t.number,\n  (n): n is t.Branded<number, { readonly PositiveNumber: unique symbol }> => n > 0,\n  'PositiveNumber'\n);\nconst result = PositiveNumber.decode(-5);\n// result will be an instance of Left since the number is not positive"}

Other packages similar to io-ts

ajv

Ajv is a JSON schema validator that provides runtime data validation using predefined JSON schemas. It is similar to io-ts in that it validates data structures at runtime, but it uses JSON schema as the basis for validation rather than TypeScript types.

joi

Joi is an object schema validation library that allows for the description and validation of JavaScript objects. It is similar to io-ts in providing runtime validation, but it uses a fluent API for schema definition and does not integrate with TypeScript types in the same way.

yup

Yup is a JavaScript schema builder for value parsing and validation. It defines a schema using a declarative API and validates objects against the schema. Like io-ts, it provides runtime validation, but it does not leverage TypeScript's type system for type definitions.

class-validator

Class-validator allows for validation of class instances based on decorators. It is similar to io-ts in that it provides runtime validation, but it is designed to work with classes and decorators, offering a different approach to defining validation rules.

README

The idea

Blog post: "Typescript and validations at runtime boundaries" by @lorefnon

A value of type Type<A, O, I> (called "runtime type") is the runtime representation of the static type A.

Also a runtime type can

• decode inputs of type I (through decode)
• encode outputs of type O (through encode)
• be used as a custom type guard (through is)

export type mixed = object | number | string | boolean | symbol | undefined | null

class Type<A, O = A, I = mixed> {
  readonly _A: A
  readonly _O: O
  readonly _I: I
  constructor(
    /** a unique name for this runtime type */
    readonly name: string,
    /** a custom type guard */
    readonly is: (v: mixed) => v is A,
    /** succeeds if a value of type I can be decoded to a value of type A */
    readonly validate: (input: I, context: Context) => Either<Errors, A>,
    /** converts a value of type A to a value of type O */
    readonly encode: (a: A) => O
  ) {}
  /** a version of `validate` with a default context */
  decode(i: I): Either<Errors, A>
}

Note. The Either type is defined in fp-ts, a library containing implementations of common algebraic types in TypeScript.

Example

A runtime type representing string can be defined as

import * as t from 'io-ts'

export class StringType extends t.Type<string> {
  // equivalent to Type<string, string, mixed> as per type parameter defaults
  readonly _tag: 'StringType' = 'StringType'
  constructor() {
    super(
      'string',
      (m): m is string => typeof m === 'string',
      (m, c) => (this.is(m) ? t.success(m) : t.failure(m, c)),
      t.identity
    )
  }
}

A runtime type can be used to validate an object in memory (for example an API payload)

const Person = t.type({
  name: t.string,
  age: t.number
})

// validation succeeded
Person.decode(JSON.parse('{"name":"Giulio","age":43}')) // => Right({name: "Giulio", age: 43})

// validation failed
Person.decode(JSON.parse('{"name":"Giulio"}')) // => Left([...])

TypeScript compatibility

The stable version is tested against TypeScript 2.7.x+

Error reporters

A reporter implements the following interface

interface Reporter<A> {
  report: (validation: Validation<any>) => A
}

This package exports two default reporters

• PathReporter: Reporter<Array<string>>
• ThrowReporter: Reporter<void>

Example

import { PathReporter } from 'io-ts/lib/PathReporter'
import { ThrowReporter } from 'io-ts/lib/ThrowReporter'

const result = Person.decode({ name: 'Giulio' })

console.log(PathReporter.report(result))
// => ['Invalid value undefined supplied to : { name: string, age: number }/age: number']

ThrowReporter.report(result)
// => throws 'Invalid value undefined supplied to : { name: string, age: number }/age: number'

Community

• io-ts-types - A collection of runtime types and combinators for use with io-ts
• io-ts-reporters - Error reporters for io-ts
• geojson-iots - Runtime types for GeoJSON as defined in rfc7946 made with io-ts
• graphql-to-io-ts - Generate typescript and cooresponding io-ts types from a graphql schema

TypeScript integration

Runtime types can be inspected

This library uses TypeScript extensively. Its API is defined in a way which automatically infers types for produced values

Note that the type annotation isn't needed, TypeScript infers the type automatically based on a schema.

Static types can be extracted from runtime types using the TypeOf operator

type IPerson = t.TypeOf<typeof Person>

// same as
type IPerson = {
  name: string
  age: number
}

Implemented types / combinators

import * as t from 'io-ts'

Type	TypeScript	Runtime type / combinator
null	null	t.null or t.nullType
undefined	undefined	t.undefined
string	string	t.string
number	number	t.number
boolean	boolean	t.boolean
any	any	t.any
never	never	t.never
object	object	t.object
integer	✘	t.Integer
array of any	Array<mixed>	t.Array
array of type	Array<A>	t.array(A)
dictionary of any	{ [key: string]: mixed }	t.Dictionary
dictionary of type	{ [K in A]: B }	t.dictionary(A, B)
function	Function	t.Function
literal	's'	t.literal('s')
partial	Partial<{ name: string }>	t.partial({ name: t.string })
readonly	Readonly<T>	t.readonly(T)
readonly array	ReadonlyArray<number>	t.readonlyArray(t.number)
type alias	type A = { name: string }	t.type({ name: t.string })
tuple	[ A, B ]	t.tuple([ A, B ])
union	A | B	t.union([ A, B ]) or t.taggedUnion(tag, [ A, B ])
intersection	A & B	t.intersection([ A, B ])
keyof	keyof M	t.keyof(M)
recursive types	see Recursive types	t.recursion(name, definition)
refinement	✘	t.refinement(A, predicate)
exact types	✘	t.exact(type)
strict types (deprecated)	✘	t.strict({ name: t.string })

Recursive types

Recursive types can't be inferred by TypeScript so you must provide the static type as a hint

// helper type
type ICategory = {
  name: string
  categories: Array<ICategory>
}

const Category = t.recursion<ICategory>('Category', self =>
  t.type({
    name: t.string,
    categories: t.array(self)
  })
)

Tagged unions

If you are encoding tagged unions, instead of the general purpose union combinator, you may want to use the taggedUnion combinator in order to get better performances

const A = t.type({
  tag: t.literal('A'),
  foo: t.string
})

const B = t.type({
  tag: t.literal('B'),
  bar: t.number
})

// the actual presence of the tag is statically checked
const U = t.taggedUnion('tag', [A, B])

Refinements

You can refine a type (any type) using the refinement combinator

const Positive = t.refinement(t.number, n => n >= 0, 'Positive')

const Adult = t.refinement(Person, person => person.age >= 18, 'Adult')

Exact types

You can make a runtime type alias exact (which means that only the given properties are allowed) using the exact combinator

const Person = t.type({
  name: t.string,
  age: t.number
})

const ExactPerson = t.exact(Person)

Person.decode({ name: 'Giulio', age: 43, surname: 'Canti' }) // ok
ExactPerson.decode({ name: 'Giulio', age: 43, surname: 'Canti' }) // fails

Strict types (deprecated)

Note. This combinator is deprecated, use exact instead.

You can make a runtime type strict (which means that only the given properties are allowed) using the strict combinator

const Person = t.type({
  name: t.string,
  age: t.number
})

const StrictPerson = t.strict(Person.props)

Person.decode({ name: 'Giulio', age: 43, surname: 'Canti' }) // ok
StrictPerson.decode({ name: 'Giulio', age: 43, surname: 'Canti' }) // fails

Mixing required and optional props

You can mix required and optional props using an intersection

const A = t.type({
  foo: t.string
})

const B = t.partial({
  bar: t.number
})

const C = t.intersection([A, B])

type CT = t.TypeOf<typeof C>

// same as
type CT = {
  foo: string
  bar?: number
}

Custom types

You can define your own types. Let's see an example

import * as t from 'io-ts'

// represents a Date from an ISO string
const DateFromString = new t.Type<Date, string>(
  'DateFromString',
  (m): m is Date => m instanceof Date,
  (m, c) =>
    t.string.validate(m, c).chain(s => {
      const d = new Date(s)
      return isNaN(d.getTime()) ? t.failure(s, c) : t.success(d)
    }),
  a => a.toISOString()
)

const s = new Date(1973, 10, 30).toISOString()

DateFromString.decode(s)
// right(new Date('1973-11-29T23:00:00.000Z'))

DateFromString.decode('foo')
// left(errors...)

Note that you can deserialize while validating.

Tips and Tricks

Is there a way to turn the checks off in production code?

No, however you can define your own logic for that (if you really trust the input)

import * as t from 'io-ts'
import { Either, right } from 'fp-ts/lib/Either'

const { NODE_ENV } = process.env

export function unsafeDecode<A, O>(value: t.mixed, type: t.Type<A, O>): Either<t.Errors, A> {
  if (NODE_ENV !== 'production' || type.encode !== t.identity) {
    return type.decode(value)
  } else {
    // unsafe cast
    return right(value as A)
  }
}

// or...

import { failure } from 'io-ts/lib/PathReporter'

export function unsafeGet<A, O>(value: t.mixed, type: t.Type<A, O>): A {
  if (NODE_ENV !== 'production' || type.encode !== t.identity) {
    return type.decode(value).getOrElseL(errors => {
      throw new Error(failure(errors).join('\n'))
    })
  } else {
    // unsafe cast
    return value as A
  }
}

Union of string literals

Use keyof instead of union when defining a union of string literals

const Bad = t.union([
  t.literal('foo'),
  t.literal('bar'),
  t.literal('baz')
  // etc...
])

const Good = t.keyof({
  foo: null,
  bar: null,
  baz: null
  // etc...
})

Benefits

• unique check for free
• better performance
• quick info stays responsive

Known issues

VS Code might display weird types for nested types

const NestedInterface = t.type({
  foo: t.string,
  bar: t.type({
    baz: t.string
  })
})

type NestedInterfaceType = t.TypeOf<typeof NestedInterface>
/*
Hover on NestedInterfaceType will display

type NestedInterfaceType = {
    foo: string;
    bar: t.TypeOfProps<{
        baz: t.StringType;
    }>;
}

instead of

type NestedInterfaceType = {
  foo: string;
  bar: {
    baz: string
  }
}
*/

Solution: the clean and alias functions

The pattern

// private runtime type
const _NestedInterface = t.type({
  foo: t.string,
  bar: t.type({
    baz: t.string
  })
})

// a type alias using interface
export interface NestedInterface extends t.TypeOf<typeof _NestedInterface> {}

//
// Two possible options for the exported runtime type
//

// a clean NestedInterface which drops the kind...
export const NestedInterface = t.clean<NestedInterface, NestedInterface>(_NestedInterface)
/*
NestedInterface: t.Type<NestedInterface, NestedInterface, t.mixed>
*/

// ... or an alias of _NestedInterface which keeps the kind
export const NestedInterface = t.alias(_NestedInterface)<NestedInterface, NestedInterface>()
/*
t.InterfaceType<{
    foo: t.StringType;
    bar: t.InterfaceType<{
        baz: t.StringType;
    }, t.TypeOfProps<{
        baz: t.StringType;
    }>, t.OutputOfProps<{
        baz: t.StringType;
    }>, t.mixed>;
}, NestedInterface, NestedInterface, t.mixed>
*/

// you can also alias the props
interface NestedInterfaceProps extends t.PropsOf<typeof _NestedInterface> {}
export const NestedInterface = t.alias(_NestedInterface)<NestedInterface, NestedInterface, NestedInterfaceProps>()
/*
const NestedInterface: t.InterfaceType<NestedInterfaceProps, NestedInterface, NestedInterface, t.mixed>
*/

Changelog

1.1.5

• Bug Fix
  • partial combinator should preserve additional properties while encoding, fixes #179 (@gcanti)
• Polish
  • use useIdentity when possible (@gcanti)